18 Jul 2017

About 570 million
years ago, large, frond-like creatures suddenly invaded the ocean floors. For
over a billion years, the Earth’s oceans were filled with bacteria and
microscopic algae, but during the Ediacaran period, from 635 to 541 million
years ago, larger multicellular organisms began crowding the seas.

Fossil imprints from
the Ediacaran derive from soft-bodied organisms resembling modern-day sea
anemones (Cyclomedusa), annelid worms
(Dickinsonia) and sea pens (rangeomorphs
such as Charnia). Among these bizarre
creatures, the rangeomorphs are the most abundant in the fossil record—and also
some of the largest.

Artist impression of rengeomorphs (credit: Jennifer Hoyal Cuthill)

Rangeomorphs were unlike
any creature on Earth today. Some were as small as a coin, while others could
grow up to 2 meters high. They looked like ferns, with branches spreading out
from a central stem, but they likely fed by filtering nutrients from the water,
similar to corals. Because rangeomorphs were so different from any known life
form, paleontologists still don’t agree whether they were primitive animals related
to soft corals, some sort of weird fungus or even a new (now extinct) kingdom
of life, the Vendobiota.

These ocean dwellers eventually
disappeared after the Cambrian explosion, some 541 million years ago, when
fast-moving predators emerged (and probably ate them).

Changes in ocean chemistry

Based on the chemical signature
of ancient seawater left on rocks, geochemists think there was a sharp rise in ocean
oxygen levels soon after the end of the Gaskiers glaciation, about 580 million
years ago. These changes in the ocean chemistry could explain the appearance of
larger and more complex marine organisms—more food, bigger bodies. However, even
though this may seem quite obvious, it’s actually quite difficult to
demonstrate.

Jennifer Hoyal Cuthill
and Simon Conway Morris, from the University of Cambridge (UK) and Tokyo
Institute of Technology (Japan), used an original approach to tackle this problem.

“We wanted to see
whether the increase in body size could point to a rise in oxygen, since the
type of growth can tells us whether the animals have nutrients available or not”,
says Hoyal Cuthill.

They suspected that
Ediacaran organisms were large because they had a ‘nutrient-dependent’ type of growth,
rather than an evolutionarily new genetic makeup.

‘Seeing’ extinct creatures grow

Many organisms can’t
grow beyond a certain size, regardless of how much they eat. Humans for
example, will (unfortunately) just get fatter, not taller, because they are
genetically programmed to reach a specific maximum height. But for some
organisms nutrient availability can affect body size. This type of nutrient-dependent growth is quite
common in invertebrates and plants. Some plants will grow almost indefinitely,
as long as there are nutrients (and light) available in the environment.

But how do you measure
growth in organisms that lived nearly 600 million years ago?

This is where
rangeomorph fossils come in handy.

Hoyal Cuthill and Conway
Morris had previously worked with several rangeomorph specimens to study the
unusual body plan of these animals. During this research it dawned on them that
the rangeomorphs’ complex fractal branching shape, with larger older branches
at the bottom and smaller younger branches on top, was the key for testing the nutrient-dependent
growth hypothesis.

“It’s like looking
back at your childhood photographs and comparing your height through your old
photos up to the present day”, says Hoyal Cuthill. “We were inferring the
history of growth of a rangeomorph by looking at parts of the structure of
different ages”.

The researchers could
basically “see” in a single fossil specimen how the animals were growing during
their lifetime, by comparing the relative size and shape of younger and older
branches.

A unique rangeomorph fossil

Fossil of Charnia (Jennifer Hoyal Cuthill)

The new study focuses
on an exquisitely preserved specimen of Avalofractus
abaculus, one of the last fossils removed from the Trepassey Formation, in
Newfoundland (Canada), before strict restrictions were imposed to protect the
site (currently called Mistaken Point Ecological Reserve). Hoyal Cuthill
obtained a high-resolution cast from the Royal Ontario Museum and scanned it by
CT- microtomography, a technique which uses x-rays to make detailed digital 3D
reconstructions.

Two other specimens (Charnia masoni and an undescribed specimen
from the South Australian Museum) were also analysed based on digital
photographs.

Mathematical and
computer models comparing the surface area and the volume of younger and older
branches showed that growth gradually slowed down as rangeomorphs got bigger,
which is exactly what happens in modern organisms with nutrient-dependent
growth.

“… You’re getting less nutrients as you
get larger, so you cannot sustain the same rate of growth, and it slows down”, Hoyal
Cuthill explains.

But there was more. Nutrient
availability can also affect body shape, which is technically called
ecophenotypic plasticity. Hoyal Cuthill and Conway Morris also found that
rangeomorphs could rapidly change shape to access higher levels of oxygen in
the seawater above them, by growing into a long, tapered shape.

Nutrient-dependent
growth provides a mechanism to explain why changes in ocean chemistry caused
the appearance of these large organisms in the Ediacaran, some 30 million years
before the Cambrian explosion.

Hoyal Cuthill next
wants to investigate whether rangeomorphs really are animals, and to which modern
groups are they related to.

“Rangeomorphs are quite mysterious and
were only relatively recently discovered and identified as Precambrian
organisms”, she says. “This is an exciting time and many researchers are
looking at the biota of the Ediacaran and finding new fascinating things”.Reference: Hoyal Cuthill, Jennifer F., and Simon Conway Morris. "Nutrient-dependent growth underpinned the Ediacaran transition to large body size." Nature Ecology and Evolution (2017). DOI:10.1038/s41559-017-0222-7This article was published originally as a guest post in the PLOS Paleo Community blog with the title "Why Precambrian life got so big" on the 18-07-2017. You can read it here.